Build FAILED.

"D:\test1\003\opensim.sln" (default target) (1) ->
"D:\test1\003\addon-modules\01DivaUtils\Diva.Utils.csproj" (default target) (2)
 ->
(CoreCompile target) ->
  WebAppUtils.cs(296,44): error CS0246: The type or namespace name 'HashSet' co
uld not be found (are you missing a using directive or an assembly reference?)

    0 Warning(s)
    1 Error(s)

Time Elapsed 00:00:00.43